1. Field of the Invention
The present invention relates to a clad material comprising a metal substrate having a multiplicity of perforated openings laminated with a metal foil.
2. Description of Related Art
Conventionally, batteries which use alkaline metal such as lithium, sodium or potassium as an active material on negative electrode thereof sometimes suffer from rupture when the pressure in the batteries is abnormally elevated. To prevent the occurrence of such a rupture, safety valves which can release the pressure outside in the case that the pressure in the battery is excessively increased have been requested and accordingly various safety valves having different mechanisms have been proposed. To assure the safety that even when the battery ruptures by a chance, broken pieces or contents of the battery do not scatter outside and cause no damage on human body, the batteries must be operated at a low pressure not more than 30 kgf/cm.sup.2.
The dry-cell type battery which uses alkaline metal as the active material on negative electrode thereof is further required to have a high scalability. A Japanese laid-open publication SHO 63-285859 discloses a safety valve which can release the inner pressure of such a battery outside. In this battery, a part of the wall of the battery vessel is made thin by cold compressing using a press until the thickness of the compressed part becomes half of the original thickness of the part. Accordingly, when the inner pressure is elevated and reaches a predetermined inner pressure level, the thinned wall part ruptures and the inner pressure is released outside.
To release the inner pressure at a low pressure of not more than 30 kgf/cm.sup.2, the thinned wall part must be made considerably thin. Accordingly, during the press working to obtain such an extremely thin wall part, fine or minute cracks may occur, and once such cracks occur, the sealability of the vessel is spoiled. Although the thinned wall part is hardened with such a press working, the hardening does not occur uniformly. Accordingly, the release valve disclosed in Japanese laid-open publication SHO 63-285859 also suffers from a drawback that even when the thinned wall part is pressed to have a uniform thickness, the thinned wall part does not always rupture at a predetermined uniform pressure.
Furthermore, although an etching method has been proposed to make a part of the wall of the battery vessel thin, it is extremely difficult to uniformly control the thickness of the thinned wall part after etching and the thinned wall part is apt to suffer from pin holes. Accordingly, thinned wall parts of all battery vessels must be subject to a pin hole test for detecting the presence of pin holes.
In this manner, with the above-mentioned method, it is extremely difficult to provide the thinned wall part which has a uniform thickness so that especially on the condition that the safety valves are to be operated to release pressure at a low pressure of not more than 30 kgf/cm.sup.2, a reliable reproductivity of the pressure releasing operation cannot be achieved.
To resolve the drawbacks of the above-mentioned methods, Japanese laid-open publication HEI 5-314959 discloses a method in which a matal plate having a perforated opening and a thin metal plate adhered with each other to produce a thinned wall part having a uniform thickness and such a method provides a valve operating pressure which is not more than 30 kgf/cm.sup.2 and has a reliable reproductivity on a pressure releasing operation.
In this method, however, since the perforated metal plate and the thinned metal plate are heated in a vacuum furnace and heat-sealed with each other under pressure, the materials for these metal plates must meet a condition that they can be heat-sealed under pressure. Namely, the materials for these metal plates are restricted to same metals or metals having similar physical properties such as a melting point. In Japanese laid-open publication HEI 5-314959, stainless steel, iron, nickel and the like are proposed as preferable materials for these metal plates.
Furthermore, to heat seal these metal plates under pressure to obtain a uniform adhering strength, an oxide film formed on the surface of these metal plates must be removed by buffing or the like and then the metal plates must be heated at a high temperature of approximately 1000.degree. C. so that the method necessitates a sophistiated operation and facilities. Furthermore, these thin metal plates are usually produced by a cold rolling so that they are subjected to hardening by working. Since the above-mentioned metal plates which are hardened by working are annealed at a high temperature during the heat sealing under pressure, the mechanical strength of these metal plates differs before and after the heat sealing operation. Accordingly, the properties of the materials before heat sealing, the heating temperature and the heating time have to be strictly controlled to make the mechanical strength (the limit strength which causes a rupture when the inner pressure is elevated) of the metal plates constant after heat sealing.